A video signal typically includes vertical display intervals, or fields, having a plurality of horizontal line intervals, e.g. 262.5 lines per field in NTSC video systems. The beginning of each vertical and horizontal interval is identified by respective vertical and horizontal sync pulses that are included in a composite video signal. During a portion of each vertical interval, information in the video signal may not be intended for display. For example, a vertical blanking interval spans approximately the first 20 horizontal line intervals in each field. In addition, several line intervals adjacent to the vertical blanking period, e.g. line 21, may be within an overscan region of a video display and will not be visible.
The lack of displayed image information during blanking and overscan intervals makes it possible to insert an auxiliary information component, e.g. teletext or closed caption data, into these intervals. Standards such as Federal Communications Commissions (FCC) Regulations define the format for each type of auxiliary information including the positioning of the information within a vertical interval. For example, the present closed captioning standard (see e.g. 47 CFR .sctn..sctn.15.119 and 73.682) specifies that digital data corresponding to ASCII characters for closed captioning must be in line 21 of field 1. Future modifications to the standard may permit auxiliary information such as closed caption data to be located in other lines, e.g. line 21 of every field.
Auxiliary video information is extracted from the video signal using a decoder. An important part of a decoder is the data slicer. The data slicer may be a voltage comparator having a video signal carrying auxiliary video information applied to one input. For optimum performance, a reference or "slicing" voltage at a second input of the comparator should be at the midpoint of the peak to peak excursion of the auxiliary video information signal. The output of the comparator would then provide a binary signal representative of the auxiliary information contained in the video signal.
A constant slicing level may not be adequate for all video signals. Video signal levels may vary depending on the source of the video signal. Utilizing a constant slicing level with varying video signal levels may bias the extracted data undesirably toward logic 0 or logic 1 resulting in erroneous data extraction. For example, if the video signal range is 0 IRE to 20 IRE, a slicing level of 10 IRE is desirable while for a video signal range of 0 IRE to 50 IRE, a slicing level of 25 IRE is desirable. If 25 IRE were used as a slicing level for a signal range of 0 IRE to 20 IRE, a logic 1 would never be extracted because the signal never exceeds the slicing level. Thus, it is desirable to adapt the slicing level to the amplitude of the input video signal.
The format of an auxiliary information component such as closed caption data includes provisions to facilitate an adaptive slicing level function. As specified in the FCC standards, a closed caption signal in line 21 begins after the "back porch" interval of the video signal with a 7 cycle burst of a sinusoidal reference waveform designated the "run-in clock" (RIC). The RIC reference component of the auxiliary video data signal is followed in the latter half of the line 21 interval by a data signal component that represents the actual closed caption data. The closed caption data standard establishes that the amplitude of the RIC signal is identical to the amplitude of the data signal. Thus, the average of the RIC signal amplitude is an appropriate slicing level for the subsequent data signal.
One method for developing the slicing voltage is to integrate the sinusoidal RIC signal and use the resulting DC voltage as the bias for the data slicer. A large integrator capacitor may be required to prevent the slicing voltage from changing due to leakage currents discharging the capacitor during the interval between occurrences of auxiliary video data (e.g. the interval from one line 21 to the next for closed caption data). A large capacitor, however, requires long integration intervals to respond to changes in video signal levels when changing the slicing level. For example, in a closed caption signal, the RIC signal is present only for 14 .mu.s (7 cycles of 500 KHz sinewave) at 33.3 ms intervals (period of one frame of video). A large capacitor may require a response time on the order of one second to respond to sudden changes in the signal level. A significant amount of auxiliary video information during the response interval may be undetected.
It may be desirable to include an auxiliary video information decoder in a video signal processing integrated circuit (IC). A large integrating capacitor may, however, be too large to be included in the IC. An extra IC pin would be required for connecting an external integrating capacitor.
Although it may be possible to design a fast integrator with component values that are small enough to be included in an IC, the resulting design may exhibit tight tolerances which may be impractical for an integrated circuit design. More specifically, IC parameters may vary during production. A design having tight tolerances may be incompatible (produce unexpected or undesirable performance) as a result of parameter variations during IC production.